MOSFETs have become the preferred devices for switching currents in numerous fields, including the computer and automotive industries. Three of the principal characteristics of MOSFETs are their gate drive voltage, their on-resistance (Rds-on) and their avalanche breakdown voltage (VB). The gate drive voltage is determined primarily by the gate oxide thickness; the thinner the gate oxide, the lower the gate drive voltage. However, a thinner gate oxide leads to a lower breakdown voltage, especially for trench power MOSFETs. The breakdown voltage is normally provided largely by a lightly-doped “drift” region that is located between the drain and body regions of the MOSFET. For example, in MOSFET 10 shown in FIG. 1, a lightly-doped N-epitaxial (epi) layer 104 is grown on a heavily-doped N+ substrate 102, which serves as the drain of the device. (Note that FIG. 1 is not drawn to scale; for example, substrate 102 would typically be much thicker than epi layer 104.) A trench is formed in the top surface of epi layer 104, frequently using a reactive ion etch (RIE) process. The walls of the trench are lined with a gate oxide layer 112, and the trench is filled with a conductive material, often doped polycrystalline silicon (polysilicon), which serves as a gate electrode 110. The top portion of the epi layer 104 is implanted with a P-type impurity such as boron to form a P-body region 108, and using appropriate photoresist masks, N and P type dopants are implanted and diffused to form N+ source regions 110 and P+ body contact regions 118 at the surface of epi layer 104. The implantations used to form P-body region 108, N+ source regions 110 and P+ body contact region 118 are frequently performed before the trench is etched.
A borophosphosilicate layer 116 is deposited and patterned so that it covers and isolates the gate electrode 110, and a metal layer 114 is deposited over the top surface of the device. Metal layer 114, which can be an aluminum or copper alloy, makes an ohmic electrical contact with N+ source regions 110 and P+ body contact regions 118.
Current flows vertically through MOSFET 10 from the N+ drain 102 and through an N-drift region 106 and a channel region (denoted by the dashed lines) in P− body region 108 to the N+ source regions 110.
The trench is typically made in the form of a lattice that creates a number of MOSFET cells. In a “closed cell” arrangement, the MOSFET cells may be hexagonal, square or circular. In an “open cell” arrangement, the cells are in the form of parallel longintudinal stripes.
When MOSFET 10 is reverse-biased, the N+ drain region 102 is biased positively with respect to the N+ source regions 110. In this situation, the reverse bias voltage appears mainly across the PN junction 120 that separates N-drift region 106 and P-body region 108. N-drift region 106 becomes more and more depleted as the reverse bias voltage increases. When the depletion spreading reaches the boundary between N+ substrate 102 and N-drift region 106, any further increases in the reverse bias are seen at PN junction 120. Thus making N-drift region 106 thicker generally provides greater protection against breakdown. Furthermore, there is a generally inverse relationship between the avalanche breakdown voltage of PN junction 120 and the doping concentration of N-drift region 106, i.e, the lower the doping concentration of N-drift region 106, the higher the breakdown voltage VB of PN junction 120. See Sze, Physics of Semiconductor Devices, 2nd Ed., page 101, FIG. 26, which provides a graph showing the relationship between the doping concentration and VB for several semiconductor materials.
Thus, to increase the breakdown voltage of junction 120, one would like to reduce the doping concentration of N-drift region 106. This in turn, however, reduces the quantity of charge in N-drift region 106 and accelerates the effect of depletion spreading. One solution would be to increase the thickness of N-drift region 106, but this tends to increase the on-resistance of MOSFET 10.
U.S. Pat. No. 5,216,275 describes a high voltage drift structure useful for trench power MOSFETs, diodes, and bipolar transistors. The drift structure includes a “composite buffer layer” that contains alternately arranged areas of opposite conductivity.
In low voltage and high density trench MOSFETs there is another limitation. A high field at the bottom of the gate oxide, which limits the breakdown voltage and the oxide thickness. U.S. Pat. No. 5,168,331 proposes a floating, a lightly doped P− region just below the trench gate oxide to reduce the field which it does. However, P-shield region (e.g., boron atoms) out diffuse towards the P-body, which increases Rds on and/or requires the packing density to be reduced.
The present invention overcomes these problems.